block building: once we propose a block to attestors - do not submit another

[Maddiaa0]

Overview

Current Behaviour

The sequencer will build a block, propose to the committee, then attempt to publish the block.
When running in a real scenario, the sequencer cannot publish until the slot boundary is hit. It will fail to publish the block, build another one, and then propose it to the committee again.

We want to prevent the committee seeing more than one proposal per slot.

I think this case was hit when a validator did not have funds, it would send multiple block proposals when it retried.

Notes

This description may not be the actual issue - so feel free to kill it if not

[Maddiaa0]

patched, but not optimally in #10327